1. Field of the Invention
This invention relates to a semiconductor light emitting device based on a semiconductor light emitting element such as a light emitting diode (LED), and more particularly to a semiconductor light emitting device for use in a white light source including a semiconductor light emitting element combined with phosphors.
2. Background Art
In recent years, semiconductor light emitting devices have been widely used for various light sources including illumination and display devices. In particular, realization of blue light emitting elements and ultraviolet elements made of GaN-based material has dramatically extended the application of white light emitting devices. It is to be expected that applications in LCD backlights, large screen or other displays, and illumination lamps, among others, will be further extended in the future.
Such a semiconductor light emitting device can be implemented by a combination of a semiconductor light emitting element for emitting short wavelength light and phosphors for converting the wavelength of the emission. That is, several kinds of phosphors for emitting light at different wavelengths can be mixed to obtain light emission with a predetermined spectrum.
For example, a desired emission color can be achieved by appropriately selecting a compounding ratio of three kinds of phosphors that absorb ultraviolet radiation and convert the wavelength to blue, green, and red light, respectively (e.g., Japanese Laid-Open Patent Application 2003-324215). For achieving white light emission, there exists an optimal compounding ratio of phosphors corresponding thereto. Alternatively, white light can also be achieved by irradiating blue and yellow light emitting phosphors with ultraviolet radiation and mixing the emitted blue and yellow light.
However, different kinds of phosphors have different specific gravities and grain size distributions, respectively. This causes a problem that various factors in the manufacturing process lead to the color tone variation of products and the decrease of production yield.
This problem may be addressed by, for example, using a single kind of phosphors in conjunction with a blue light emitting element. More specifically, yellow light obtained by irradiating yellow phosphors with blue light can be mixed with blue light from the light emitting element to obtain white light.
In this case, however, there is a problem that the color tone is unstable in response to the variation of ambient temperature because the temperature dependence of light emission in the light emitting element is different from the temperature dependence of wavelength conversion in the phosphors. Moreover, the combination of a semiconductor light emitting element and a single kind of phosphors is also unfavorable because the achievable color range is limited to composites of the two colors.